The increasing need for high bit-rate data transmissions through optical fibers presents significant challenges to the fiber-optic telecommunications industry. For example, high bit-rate systems are highly susceptible to optical fiber nonlinearities. One option to confront these challenges is to transmit the data in the Differential-Phase Shift Keying (DPSK) modulation format. The DPSK modulation format is a well known digital modulation format that conveys data by modulating the phase of the carrier wave. The DPSK modulation format is compatible with both RZ and NRZ data formats at high data rates.
The DPSK modulation format has numerous advantages over the more standard On-Of-Keying (OOK) modulation format. One advantage of using the DPSK modulation format is that, when DPSK signals are detected using a balanced receiver, the OSNR sensitivity is improved by up to 3 dB. See, for example, A. H. Gnauck and P. J. Winzer, “Optical Phase-Shift-Keyed Transmission,” IEEE Journal of Lightwave Technology, vol. 23, pp. 115-30, 2005. Another advantage of using the DPSK modulation format over the OOK modulation format is that the DPSK modulation format is more tolerant to fiber optic non-linearities, such as self-phase modulation.
However, the DPSK modulation format is more complicated than the OOK modulation format. In DPSK systems, the digital information is written in the optical phase of the signal and, therefore, the digital information cannot be detected by ordinary intensity detectors. Consequently, DPSK receivers typically include optical demodulators, which convert the phase modulated signal to an amplitude modulated signal. The resulting amplitude modulated signal can then be detected by ordinary optical power detectors.
For most optical networks, chromatic dispersion is a major limitation to the distance of the fiber optical links and determines the complexity of network. Chromatic dispersion is a well known effect in all fiber optic systems that causes inter-symbol interference. In most cases, dispersion effects are a direct function of the transmitted signal bandwidth. The higher the bandwidth, the higher the link penalty due to chromatic dispersion. DPSK modulation, in particular, has a larger bandwidth compared to most conventional data formats and, therefore, has a stringent tolerance to fiber chromatic dispersion.